Mason and University of Chicago psychologists Jean Decety and Inbal Ben-Ami Bartal describe their rat empathy-testing apparatus: An enclosure into which pairs of rats were placed, with one roaming free and the other restrained inside a plastic tube. It could only be opened from the outside, which is exactly what the free rats did — again and again and again, seemingly in response to their trapped companions’ distress.

Once rats learned to free their trapped and agitated partners, they did so almost immediately in trial after trial. The behavior was clearly deliberate. When the restrainer was empty, rats ignored it. When stuffed rats were restrained, the rats ignored them. “It’s compelling evidence that it’s the distress of the trapped cagemate motivating this helping behavior,”

To make sure the rats weren’t responding to some immediate social reward — a rat version of a thank-you hug — the researchers tweaked the apparatus so that trapped rats were released into a separate cage. Again, the rats freed each other. When given the opportunity to eat chocolate treats first, rats were as likely to release their companions first, and even shared the chocolate with them.

According to de Waal, the results “show for the first time that rodents are not just affected by the emotions of others, but that empathy motivates altruism.” He believes the rats responded to an instinctive urge to make their compatriots feel better, just as humans and chimpanzees and some cetaceans do. “The mechanism must ancient,” said de Waal.

For Bartal, whether rats were motivated by their companions’ distress or their own is less interesting than the simple fact they responded at all. “The bottom line here is that nature is very smart. Nature made it rewarding for us to end the suffering of another,” she said.

With a few liberating swipes of their paws, a group of research rats freed trapped labmates and raised anew the possibility that empathy isn’t unique to humans and a few extra-smart animals, but is widespread in the animal world.

Though more studies are needed on the rats’ motivations, it’s at least plausible they demonstrated “empathically motivated pro-social behavior.” People would generally call that helpfulness, or even kindness.

“Rats help other rats in distress. That means it’s a biological inheritance,” said neurobiologist Peggy Mason of the University of Chicago. “That’s the biological program we have.”

In a study published Dec. 7 in Science, Mason and University of Chicago psychologists Jean Decety and Inbal Ben-Ami Bartal describe their rat empathy-testing apparatus: An enclosure into which pairs of rats were placed, with one roaming free and the other restrained inside a plastic tube. It could only be opened from the outside, which is exactly what the free rats did — again and again and again, seemingly in response to their trapped companions’ distress.

The experiment built on research conducted several years ago by geneticist Jeff Mogil at McGill University, where mice were shown capable of “emotional contagion” — a slightly scary-sounding term denoting a tendency to become upset when cagemates were in pain. This might not seem surprising, but anecdotes from wild animal observations don’t pass academic scrutiny, and it hadn’t before been shown in captive mice. It hinted at unexpectedly sophisticated cognition: Mice were supposed to feel pain, but not each other’s, at least not outside children’s stories.

At the time, ethologist Frans de Waal of Emory University, whose work has helped redefine what’s known scientifically about thoughts and feelings in chimpanzees and dolphins and elephants, said Mogil’s experiment “justifies speaking of ‘empathy’” — the ability to both put oneself in the shoes, or paws, of another, and to become emotionally involved in their situation. Sure, mice almost certainly weren’t so empathic as humans, but maybe they had the seeds of it. Maybe empathy wasn’t the result of some high-powered cognitive process, as most biologists and psychologists preferred to think, but a relatively simple phenomenon.

Wrote de Waal in Scientific American, “This mouse experiment suggests that the emotional component of this process is at least as old as the mammals and runs deep within us.”

Still, it was hard to know what to think, and emotional contagion didn’t equal empathy. Maybe the mice were simply fearful for themselves. But the possibility was open for investigation. And around the same time as the McGill studies, Bartal — then researching cancer in Israel — noticed rats at her lab becoming distressed when surgeries were performed on other rats. She couldn’t shake the feeling that empathy was involved. When she read about a rat bringing food to a trapped rat, she again thought about empathy.

Bartal went to the University of Chicago, where she joined with Decety, a leading scholar on empathy and prosocial behavior, and Mason, who’d been intrigued by Mogil’s work. Together they designed the new study — and not only did they find what might be empathy, but the rats acted on it.

Once rats learned to free their trapped and agitated partners, they did so almost immediately in trial after trial. The behavior was clearly deliberate. When the restrainer was empty, rats ignored it. When stuffed rats were restrained, the rats ignored them. “It’s compelling evidence that it’s the distress of the trapped cagemate motivating this helping behavior,” said Mason. “It is a huge leap up to use emotional contagion to actually do something, to actually help another individual.”

To make sure the rats weren’t responding to some immediate social reward — a rat version of a thank-you hug — the researchers tweaked the apparatus so that trapped rats were released into a separate cage. Again, the rats freed each other. When given the opportunity to eat chocolate treats first, rats were as likely to release their companions first, and even shared the chocolate with them.

“Empathy is a truly powerful motivator, on a par with the desire for chocolate!” said de Waal, who was not involved in the new study.

According to de Waal, the results “show for the first time that rodents are not just affected by the emotions of others, but that empathy motivates altruism.” He believes the rats responded to an instinctive urge to make their compatriots feel better, just as humans and chimpanzees and some cetaceans do. “The mechanism must ancient,” said de Waal.

However, the researchers stopped short of ascribing the results to a conclusive display of empathy. It’s possible the rats were less concerned with alleviating the suffering of brethren than soothing their own upset feelings. Perhaps the trapped rats’ distress calls were simply loud and annoying, and the free rats wanted to quiet them. One potentially important experimental condition — the opportunity for free rats to simply leave — wasn’t tested.

“The reservation I have is that it’s very difficult to demonstrate empathy. You have to show that the animal is putting itself in another’s shoes, and I’m not sure that’s demonstrated here,” said Joshua Plotnik, an Emory University psychologist and collaborator with de Waal. But Plotnik still called the observations “very exciting.”

'Nature made it rewarding for us to end the suffering of another.'

According to Mason, further tests are planned in which rats’ stress responses will be damped by drugs. If a rat feels no distress itself but still frees a trapped companion, or if a trapped rat expresses no distress but is still rescued, empathy will seem more likely. “We can figure this question out. It’s completely tractable,” said Mason. “And this experimental model is unbelievably easy to set up. It’s our fervent hope this model will be used by many people to look at helping behavior.”

Cognitive mechanisms thought to underlie empathy and helpfulness could be tested, Mason said. So could the effects of personality traits, sex differences — females rats seemed more helpful, which tracks with studies of chimps and humans — or genetic and environmental variables. Indeed, the tests needn’t be restricted to rats, but could involve any species amenable to captivity.

For Bartal, whether rats were motivated by their companions’ distress or their own is less interesting than the simple fact they responded at all. “The bottom line here is that nature is very smart. Nature made it rewarding for us to end the suffering of another,” she said.

While the researchers didn’t discuss mechanisms underlying the possible empathy, Bartal and de Waal suspect it’s linked to the lengthy care and nursing provided, as in all mammals, by mother rats. “Mammals that need nurture and care after they’re born would require some form of empathic connection between mother and offspring,” Bartal said. Sociality could be another important factor. Rats live in large family groups with complex hierarchies, and empathy is especially important in social settings.

Rats also share basic neurological features, such as a highly developed limbic system and various hormones and neurotransmitters, with all other mammals. These could provide a common ancestral origin for empathy, said Bartal, or evolution could have shaped them independently in converging ways. All roads could lead to empathy.

Of course, mammals don’t have a monopoly on intelligence or sociality or maternal care. Octopi are extraordinarily smart. So are many birds, which also care for their young and can live in large colonies. The seeds of empathy, if that’s what the rats have, could be scattered widely. “Nature has an interesting way of using different structures for similar functions,” said Bartal.